In certain applications, steel parts are manufactured that combine high mechanical strength, high impact strength and good corrosion resistance. This type of combination is particularly desirable in the automobile industry, where attempts are being made to significantly reduce the weight of the vehicles. This weight reduction can be achieved with the use of steel parts with very high mechanical characteristics and a martensitic or bainitic-martensitic microstructure. Anti-intrusion and structural parts, as well as other parts that contribute to the safety of automotive vehicles such as: bumpers, door or center pillar reinforcements and wheel arms, for example, require the above mentioned characteristics, for example. The thickness of these parts is preferably less than 3 millimeters.
EP0971044 also describes the fabrication of a steel sheet coated with aluminum or an aluminum alloy, the composition of which includes, expressed in percent by weight: 0.15-0.5% C, 0.5-3% Mn, 0.1-0.5% Si, 0.011% Cr, Ti<0.2%, Al<0.1%, P<0.1%, S<0.05%, 0.0005%<B<0.08%, the remainder being iron and the inevitable impurities resulting from processing. This sheet is heated to achieve an austenitic transformation and then hot stamped to fabricate a part, which is then cooled rapidly to obtain a martensitic or martensite-bainite structure. In this manner, it is possible to achieve a mechanical strength greater than 1500 MPa, for example.
An additional known fabrication method is called “ausforming”, in which a steel is completely austenitized and then rapidly cooled to an intermediate temperature, generally around 700-400° C., a range in which the austenite is metastable. This austenite is hot-shaped and then rapidly cooled to obtain a totally martensitic structure. Patent GB 1,080,304 also describes the composition of a steel sheet intended to be used with a method of the type described above which contains 0.15-1% C, 0.25-3% Mn, 1-2.5% Si, 0.5-3% Mo, 1-3% Cu, 0.2-1% V.
GB 1,166,042 likewise describes a steel composition suitable for this ausforming process which contains 0.1-0.6% C, 0.25-5% Mn, 0.5-2% Al, 0.5-3% Mo, 0.01-2% Si, 0.01-1% V.
These steels include significant additions of molybdenum, manganese, aluminum, silicon and/or copper. The purpose of these elements is to create a wider range of metastability for the austenite, i.e. to retard the beginning of the transformation of the austenite into ferrite, bainite or pearlite, at the temperature at which the hot-shaping is carried out. The majority of these studies devoted to ausforming were performed on steels that have a carbon content greater than 0.3%. Therefore, these compositions that are suitable for ausforming have the disadvantage that particular precautions must be taken for welding, and they also present particular problems if a hot-dip coating is to be applied. These compositions also include expensive alloy elements.